When assigning an experience to a class, you are only inviting the students who were part of the class at the time of the assignment. If you add students to a class after you assign, or you only invited a certain subset of students to an assignment initially, you can add students to the assignment. Here's how:
Click on the Assigned Experience
Click on the Dashboard (default when you click assignment)
Click on Students in the top bar
Then click Add Students to add the student(s) that you want to invite. If you are teaching an experience that contains small group work, you will also need to choose a small group for the student to work in.
Removing Students from an Assigned Experience
If you want to remove any student from an experience, click the X on the far right. As long as the student has not started, you can remove the student with this action.